Flexible recyclable solar cells are the most recent addition to solar cell research, and have the potential to lower the costs of solar cells while eliminating the need for the petroleum-based components of traditional solar cells. Zhou et al.(2013) studied the power conversion efficiency, the rectification in the absence of light, and the recyclability of polymer solar cells created with cellulose nanocrystal (CNC) substrates. CNC’s are extracted from plant fibers, and are more environmentally attractive due to their ability to be recycled than their petroleum-based counterparts. Zhou et al. found the power conversion efficiency (PCE) of solar cells on CNC substrates to be noticeably higher than previous attempts at solar cells on paper-like substrates. The CNC substrates and solar cells were also found to be separable when dispersed in distilled water at room temperature, allowing for the various components to recovered and recycled.
Zhou, Yinhua, Fuentes-Hernandez, Canek, Kahn, Talha M., Liu, Jen-Chieh, Hsu, James, Shim, Jae Won, Dindar, Amir, Youngblood, Jeffrey P., Moon, Robert J., Kippelen, Bernard. 2013. Recyclable organic solar cells on cellulose nanocrystal substrates. Scientific Reports 3, 1536
Zhou et al. found their CNC substrate-based solar cells to have a 2.7% PCE; they attribute the low PCE to both the limited transmittance of the thin Ag layer which served as the bottom electrode and to the uneven and random distribution of the CNC in the clear CNC film. The CNC film is desired to be as transparent as possible, while intensifying the light onto the detector. However, the inconsistencies in the spread of the CNC’s (which are only a few hundred nanometers long) cause scattering of the light, lowering the intensity of the light hitting the detector. The efficiency of the solar cell is determined by the intensity of the light hitting the detector, and the transmittances of CNC substrates were found to be lower than that of glass.
The other piece of the study conducted by Zhou et al. examined the recyclability potential of the CNC substrate solar cells they created. It was found that the CNC substrate solar cells could be separated into their major components (substrate, organic and inorganic materials) by immersing them in distilled water at room temperature and running the solution through several simple filtering processes. Immersing the solar cell in water disintegrates the CNC substrate, allowing the solid wastes to be filtered out with a simple paper filter. The photoactive layer could then be separated from the electrodes by rinsing the solid wastes over filter paper using chlorobenzene, leaving the materials which acted as electrodes behind.
Zhou et al. theorize that the CNC substrate could be tweaked and modified to decrease the scattering of the light, increasing the PCE of the solar cell. In addition, they propose that using a different or modified material for the bottom electrode would increase the transmittance.